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Excursion guide
Joakim Mansfeld
June 2012
J. Mansfeld: The geology of Utö – excursion guide
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The geology of Utö
Introduction
Utö exhibits an unique environment in many ways.
In the earlier years it was the iron mines that was
important, one reason being that they were the mines
(of a reasonable size) that were located closest
to Stockholm. More than 200 years ago Utö also
became interesting to science because of its unusual
rock types and richness in rare minerals. Utö also
played a roll in the earlier years in the history of
how the elements were discovered. Today Utö is still
known for its unusual mineralogy and its interesting
bedrock, but maybe more for it’s easily accessible
localities and beautiful environments.
The formation of Utö
The bedrock of Utö is part of a large region with
volcanic and sedimentary rocks formed around 1900
million years ago (Ma). These rocks was intruded
by a old generation of granitoids, more or less
contemporaneous with the volcanism. On Utö rocks
of this generation comprise felsic volcanic rocks,
greywackes and limestones, but very little granitoids.
The formation of these rocks was probably the result
of a converging plate boundary with a subduction
zone to the present southeast of Utö and with a
down-plunging direction towards the northwest.
This subduction setting created the rocks of the
Bergslagen region, a c. 200x300 km large area in the
south central part of Sweden. The name Bergslagen
derives from ”bergslag”, an ancient Swedish juridical
term describing a region with mining privilegies. In
the Bergslagen area several thousand old mines are
also known, today three is still in production, and
one is to re-open within short.
The volcanism at Utö and in Bergslagen ceased,
but the plates continued to converge and the result
was deformation and metamorphism, i.e. mountain
Simplified geological map of Bergslagen showing metavolcanic rocks (yellow), metasedimentary rocks (blue), deformed
Svecofennian intrusions (brown) and late orogenic granites (pink). Red colours to the west and southwest are rocks
belonging to the Transscandinavian Igneous Belt. Grey areas are Palaoezoic sedimentary rocks. Modified after Koistinen
et al. (2001).
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J. Mansfeld: The geology of Utö – excursion guide
Geological map of northern Utö, modified from Talbot (2008). Stars indicate the described localities.
building. At Utö the metamorphism reached
temperatures up to 700°C as evident by metamorphic
minerals, which can be found in the greywackes
(Barrientos 2011; Engström 2011). The timing
of the metamorphism is not well constrained, but
probably lies in the range 1860–1840 Ma. This also
means that most of the rocks found on Utö really
are metamorphic rocks. However, an usual practice
in Sweden is to skip the ”meta-” prefix in the rock
name. This is also done in this guide.
Some time after the peak metamorphic event, and
formation of the mountain chain, followed a crustal
collapse. We do not know exactly the mechanism
behind that, but we can see the result of it – at around
1800 Ma a second generation of granites intruded the
older rocks. These granites were probably formed by
melting of the lower crust during the collapse event.
The composition of these granites varies somewhat,
most likely due to the varying composition of
the original crust that melted. In most places in
Bergslagen this event led to the formation of a
younger generation of granites, usually referred to
as ”late orogenic” granites. The appearances of these
granites varies a lot and many typical granites can
be distinghuised in Bergslagen, and those have also
got names of their own as e.g. the Stockholm granite.
Pegmatites usually accompanies the granites. On Utö
there are no granites of this generation, but several
different kinds of pegmatites.
Pegmatites are the last rocks to form from a granite
magma and as such pegmatites act as ”trash cans”
for unusual elements that fit poorly in the crystal
structures of common magmatic minerals. The
fluids that form the pegmatites evolve as more and
more material crystallize, thus leaving a fluid with
increasing concentrations of unusual elements. The
J. Mansfeld: The geology of Utö – excursion guide
last pegmatites to form are thus also the most extreme
in their compositions, which usually is referred to
as most fractionated. On Utö there are a range of
pegmatites with different levels of fractionation,
from early formed coarse-granied pegmatites with
simple mineralogy, to the most extreme fractionated
type. The elements that are especially enriched in the
fractionated pegmatites on Utö are lithium, boron,
tantalum etc. One of the interesting minerals that can
be found in the fractionated pegmatites are elbaite,
a lithium-bearing turmaline (turmaline is a group of
cyclo-silicate minerals with boron). Elbaite forms
beautifully colouerd crystals with red, green or blue
colours.
Formation of the pegmatites was the last rockforming event on Utö. From c. 1800 Ma Utö, and
Bergslagen, was part of a stable continental area.
Plate tectonic events further away led to short events
of fracturing (sometimes accompanied by intrusion
of mafic magmas) and faulting.
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Formation of biogenic carbonates during more quiet
periods. The iron ores probably started to form
during this period when solutions rich in ferrous iron
reacted with the oxygenated sea water. The solutions
also carried silica and the result become banded iron
formations.
1900 Ma
The volcano is now at its peak. It is a large (c. 10
km in diameter?) central volcano with a history of
violent pyroclastic eruptions and subsequent caldera
collapses (e.g. Allen et al. 1996). Erosion results in
sandy greywackes and probably even more coarsegrained deposits. Formation of biogenic carbonates
and banded iron ore formations. At this point the
small sulphide deposits was also formed as a result
of seawater circulation through the volcanic pile.
1905 Ma
A small subaquatic rhyolitic volcano has formed and
its lavas and ashes mixes with and starts to overlie
pelitic greywackes. During periods of quiencence
biogenic carbonates form.
1905–1900 Ma
The volcano grows and breaches the water
surface. Formation of rhyolitic lavas, ashes and
ignimbrites. Erosion of the volcanic deposits results
in rather coarse-grained psammitic greywackes and
conglomerates. The volcano is now a central volcano
that sometimes collapses and forms calderas.
1880 Ma
The volcano was not long lived, and when volcanism
and magmatism ceased in the area the lithosphere
cooled and sank, and the area was submerged. Long
before the volcano drowned it had become extinct
and it’s top was probably eroded flat. The increasing
water depth and also increased distances to any land
area that could be eroded led to a gradual fining of
the sediments deposited.
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J. Mansfeld: The geology of Utö – excursion guide
c. 1850 Ma
Plate movements led to collisions of island arcs and
volcanic arcs, which led to metamorpism and folding
during the Svecofennian Orogeny. The bedrock at
Utö experienced a high-temperature low pressure
metamorphism with formation of andalusite, garnet
and sillimanite.
Geomorphology
Stockholm Archipelago is characterized by a old
peneplain that has been fractured and faulted. The
Cambrian peneplain surfaces can still be seen in
places, and Cambrian sandstone injections are fond
in many places, demonstrating that the amount of
erosion since the Cambrian is almost negligible.
The fracture and fault zones has, however, been
subjected to weathering during varmer and moister
climates. The weathering could in places be down
over 100 m deep. The Quaternary glaciations cleared
these weathered fractures and created broad valleys
between fresh and unweathered bedrock hills.
The rocks of Utö
1820–1800 Ma
Tectonic collapse of the Svecofennian Orogen and
heating of the lower crust caused partial melting
of the metasedimentary rocks, which on Utö led to
formation of granitic fluids. These intruded along
fractures and formed the different types of pegmatites
found here.
Present
Utö is eroded down to a small island. In the northwest
and southeast the greywackes are found. The central
part of the island is characterised by a complicately
folded assemblage of interlayered volcanic and
carbonate rocks. Here the banded iron formation and
the small sulphide deposit are also located. Different
types of pegmatites crosscut all other rock types.
Greywacke
Originated as clayey and sandy sediments (pelites
and psammites) in the deeper part of the sea and
at the flanks of the volcano. The greywackes
are sometimes very well preserved with original
sedimentary structures such as conglomerates,
bedding, graded bedding, and cross bedding (Allen et
al. 1996; Lundström & Koyi 2003). At other places
deformation and metamorphism has transformed the
greywackes to migmatites.
Felsic volcanic rock
Different varieties of volcanic rocks can be found
on utö, most of it as ashes formed during violent
eruptions, either plinian or phreatomagmatic (Allen
et al. 1996). A thick sequence of quartz porphyritic
rhyolites on the southeastern side of northern Utö
is probably a near-surface intrusion, i.e. a magma
chamber.
Marble
The central part of northern Utö is dominated by
carbonate rocks. The carbonates are inpure with
varying content of silicates. The origin of the carbonate
rocks on Utö is unknown because of the high degree
of recrystallization. At other places in Bergslagen
carbonates are known to be biochemically deposits
in the form of stromatolites. Such an origin is also
possible on Utö since the carbonate rocks seem to be
shallow-water deposits (Allen et al. 1996).
J. Mansfeld: The geology of Utö – excursion guide
Iron ore
The iron ore of Utö was a small ore, but it was
strategic important as it was located both close
to Stockholm and to the sea. The iron ore itself is
a rather common ore of BIF-type, i.e. an iron ore
consisting of millimetre thick layers of alternating
iron oxides and other minerals, mainly quartz, but
also other silicates or sometimes carbonates. On Utö
the quartz in the ore is jaspilite, a variety coloured
by iron oxides. BIF is an acronym for Banded
Iron Formation, and is globally the most important
iron ore type. Most of the Worlds BIF ores formed
during a period between 2500 and 1800 Ma, i.e.
simultaneously with a dramatically rising in oxygen
levels in the atmosphere and oceans. The BIF ores are
believed to have formed by precipitation of inslouble
ferric iron as it formed by oxidation of ferrous iron
solved in the oceans. The iron ore of Utö is thus not
that spectacular in itself. However, Utö is a very good
example that rare and spectacular sites can form by
coincidence (see below).
Pegmatite
Pegmatite is a coarse grained rock formed by
magmatic solutions that remained after crystallization
of the magmas. Pegmatites have often a mineralogy
similar to granites, e.g. quartz, potassic feldspar,
plagioclase, muscovite and biotite, but they usually
also contain a number of more rare minerals.
The most famous of Utö’s pegmatite dykes are the
one that crosscut the iron ore, just at the places of
the mines. It is of a type called lithium pegmatite (or
LCT pegmatite after the elements lithium-caesiumtantalum). LCT pegmatites are rather rare. They
are believed to have formed by partial melting of
ancient deep sea sediments (such as greywacke) in
e.g. subduction zone settings, or during regional
metamorphism. A similar pegmatite can be found at
Varuträsk outside Skellefteå in northern Sweden. The
unique with the pegmatite on Utö is that it happened
to intrude an iron ore, which is why Utö exhibits a
number of very rare minerals.
The iron mines
The iron ore of Utö has been known for a long time.
The first written documentation is from the middle of
the 17th century. There are, however, archaeological
evidences that the ore was mined already during the
13th century. The evidences are not from Utö itself,
but from Gotland. Here remnants of a furnace for iron
production was found together with ore containing
minerals unique for Utö.
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During the years 1711–1878 around 2.1 metric
tonnes of of ore were mined, and totally the mines
are estimated to have produced a total of 2.5 tonnes
(in comparison, this is about 10% of todays annual
production in Kiruna and Malmberget, the two largest
iron ores in Sweden). The last mining activity was in
1878, but the ore has been subject to later exploration
attempts, the last in the 1950th.
Today the deepest mines are Nyköpingsgruvan, with
its 214 meter, and Finngruvan, 150 meters deep.
The minerals of Utö
As mentioned above Utö hosts a large variety of
minerals, of which some are very rare. Around 70
different minerals has been identified. The minerals
can be divided into groups based on their genesis and
host rock.
Ore minerals
Two types of ores are present on Utö, banded iron
formation and a small exhalative (Zn-Pb-Ag)
sulphide deposit. The iron ore minerals are hematite
and magnetite. Amongst the sulphide minerals
galena, sphalerite, chalcopyrite, chalcosite, bornite,
arsenopyrite, molybdenite, pyrite and silver has been
found (Jonsson 1996).
Pegmatite minerals
The most characteristic of the pegmatite minerals
on Utö are the lithium bearing minerals such as
spodumene (a pyroxene), petalite (a feldsparlike mineral), lepidolite (a mica), and elbaite (a
tourmaline with four varieties; indigolite – blue,
rubellite – red, verdelite – green, and achroite
– colourless). But the fractionated pegmatites also
carries a large number of other minerals, from
major minerals such as quartz and feldspar (e.g. the
green Pb-bearing microcline variety amazonite),
muscovite and schörl (a black turmaline), to minor
phases such as beryl, fluoroapatite, cassiterite,
arsenopyrite, manganoapatite, manganotantalite,
pollucite, topaz and zircon. Rare minerals are e.g. the
beryllium minerals mentioned above together with
often secondary formed minerals as e.g. cookeite,
eucryptite, stokesite, triphyline-lithiophilite and
wickmanite (Jonsson 1996).
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J. Mansfeld: The geology of Utö – excursion guide
Minerals formed due to reactions with the lithium
pegmatites and surrounding rocks
The most famous in this group of minerals is of
course holmquistite. But other minerals formed due
to reactions between the pegmatite and the country
rock are e.g. epidote, schörl and bavenite (Jonsson
1996).
Skarn minerals
Minerals formed through reaction between carbonate
and silicate rocks during metamorphism, or through
reaction between carbonates and granitic fluids
from the pegmatites. Here minerals such as biotite,
actinolite-tremolite, diopside, epidote, fluorite,
grossular, almandine, hornblende, calcite and
serpentine belong (Jonsson 1996).
Utö’s place in the
history of science
Ores and production of metals have for several
centuries been an important part of Sweden’s
economy. It was thus natural that many of the
most famous Swedish scientists were interested in
minerals. Amongst those we e.g. the discoverer of
oxygen, Scheele (in the end of the 18th century), and
the inventor of the chemical language, Jöns Jakob
Berzelius (in the beginning of the 19th century). At
that time there were very little difference between a
chemists, a mineralogist or a geologist as the subjects
were very closely related. The difference between a
mineralogist and a chemist was practically none, since
both worked with minerals. The period comprising
the late 18th century and the early 19th century is
also characterized by the discovery of the elements
and their systematics. This is especially pronounced
in Swedish science at that time since 21 of the 90
naturally occurring elements was discovered here.
Utö, with it’s interesting geology and location close
to Stockholm, played an important part in this. The
discovery of the mineralogical richness of Utö is a
fantastic history in itself. It was a Portugese, born
in Brazil, Jose Bonifacio de Andrada e Silva that
in 1800 was the first to describe some of the more
famous minerals of Utö. He visited the island already
during the 1790’s as part of a travel in Sweden and
Norway (Smeds 1986). He described a long row
of new minerals from Utö, most of them are today
not valid mineral names, but he is responsible for
the description of lepidolite (lithium mica) and
spodumene, he also named the blue variety of
lithium turmaline (elbaite), indigolit. For the rest of
the World, Andrada (who also has been honoured
by giving his name to the garnet andradite) is most
famous for being the first president of the free nation
Brazil.
Spodumene also plays a role in the discovery of the
elements. It was in spodumene from Utö that one
of Berzelius diciples, Johan August Arfvedson, in
1817 first found and described the element lithium.
The name was taken from the Greek word ”lithos”,
meaning stone, since lithium was the first alkali metal
extracted from a mineral. The already known alkali
metals natrium and kalium had been extracted from
sea salt and plant ash respectively.
Today c. 70 different minerals are known from
Utö (Jonsson 1996). The most known of these are
the lithium minerals. The most famous minerals of
Utö is also a lithium mineral – holmquistite. It was
discovered by A. Osann during a field trip in 1910,
in conjunction with the International Geological
Congress in Stockholm and was named after the
organizer P. J. Holmquist. During later years some
rare beryllium minerals (milarite, chiavennite,
bavenite) have also been found on Utö.
J. Mansfeld: The geology of Utö – excursion guide
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Geological map of the central part of northern Utö presented at the excursion during the International
Geological Congress 1910. Blue-green – marble, orange – porphyritic volcanic rocks, yellow –
sedimentary and volcanoclastic rocks. red – pegmatite. (Holmquist 1910).
A selection of localities on Utö
Utö hotell – marble
Just outside the hotel and c. 100 m from the bridge
(Ångbåtsbryggan) a small rounded ridge of marble is
easy accessible. The surface of the rock is black and
very pitted. The pits and small groves in the marble is
karren, a weathering fenomena typical of carbonate
rocks. The rock itself is light creamy white.
Utö vacation cottages – contact between marble and
felsic volcanic rock with skarn
The central part of Utö comprises an intermingled
assemblage of volcanocalstic rocks and carbonates.
During metamorphism the carbonates and silicate
rocks have reacted with each other creating a reaction
skarn (a metasomatically formed calc-silicate rock).
At the end of a row of rental cottages a good example
of this is demonstrated. Here a contact between a
dense and very fine-grained volcanoclastic rock and
an impure marble can be seen. The contact is seen as
a c. half meter wide reaction zone with large radially
growing amphibole crystals.
Stora Persholmen – greywacke and pegmatite
On the small island of Stora Persholmen, just to the
north of the main bridge, the bedrock is composed
of deformed and metamorphosed greywackes.
Primary layering can still be seen as compositional
changes, but primary sedimentary structures are
very hard to detect. Instead deformation structures
such as folding, refolded folds, ptygmatic folding,
boudinage, and kink bands are common. In the
northeastern part of the islet the bedrock is even
more deformed and metamorphosed, and can almost
be compared with a migmatite. Large irregular dikes
and bodies of both white and red coarse-grained
pegmatites cut the greywackes. Some of the dykes
are themselves deformed pointing to at least two
generations of pegmatite intrusions. The mineralogy
of the pegmatites are, however, rather poor. Quartz,
potasium feldspar, plagioclase, muscovite, biotite
and black tourmaline (schörl) can be found.
Nyköpingsgruvan – iron ore and lithium pegmatite
We will stop on the path just between the two largest
mines, Nyköpingsgruvan and Långgruvan. The path
itself lies there because the rock left behind is part
of the lithium pegmatite cutting the iron ore. The
pegmatite can be seen as a powdery withish rock on
the southern side of the path. Another body of the
same pegmatite type can be seen as a c. 2–3 meter
wide rock bridge across the Nyköping mine.
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J. Mansfeld: The geology of Utö – excursion guide
The large waste rock dump (Stora varphögen)
Several hundred years of mining on Utö has produced
large volumes of rock waste. Most of it is found
as large heaps around the large central mines, but
smaller waste rock dumps can also be found at other
places. Today these dumps are protected as historical
remains. Collecting and scavenging in most of the
dump is thus forbidden. South of the central mines
waste rocks from many of Utös mining operations can
be found in a large leveled dump known as ”Stora
varphögen”. This large dump is seen as a exception
from the general prohibition and here it is possible
to look around and pick samples from the mines and
surrounding rocks. Most of the finds are iron ore of
differing quality, but also pegmatite and samples from
the surrounding bedrock can be found. Furthermore,
sulphide minerals from a small sulphide mine some
hundreds meter to the northeast of the large mines are
also found here.
Rävstavik – well preserved greywacke
Rävstavik is a large area of beautyful exposed outcrops
of greywacke. In contrast to Stora Persholmen,
much of the rocks here are rather well preserved and
primary sedimentary structures such as bedding, cross
bedding, graded bedding and conglomerate layers can
be seen at may places. The sediment itself is rather
coarse-grained, and psammitic layers dominate.
Ptygmatic folded quartz veins, however, reveal that
also here the amount of deformation is rather large.
In more pelitic metamorphic minerals in the form of
altered andalusite can be seen as greyish white diffuse
blobs.
Fårskärsudd – greywacke with garnet
South of Rävstavik greywackes that underlie the
sediments at Rävstavik can be found. The greywackes
here were deposited before the main volcanic phase
and are deposited in deeper water farther from land.
The greywackes are more pelitic and homogenous in
character than the sandy greywackes at Rävstavik.
The pelitic composition was also favourable for
growth of garnet in some layers.
References
Allen, R.L., Lundström, I., Ripa, M., Simeonov, A. &
Christofferson, H., 1996: Facies analysis of a 1.9 Ga
continental margin, back-arc, felsic, caldera province with
diverse Zn-Pb-Ag-(Cu-Au) sulfide and Fe oxide deposits,
Bergslagen region, Sweden. Economic Geology 91, 979–
1008.
Barrientos, N., 2011: Determination of metamorphic conditions
in metapelites on Stora Persholmen, NE Utö, with EMPA
analysis. Unpubl. batchelor thesis. Department of Geological
Sciences, Stockholm University. 29 p.
Engström, A., 2011: Investigation of the metamorphic
environment conditions of Persholmen, NE Utö, with SEM
generated data. Unpubl. batchelor thesis. Department of
Geological Sciences, Stockholm University. 21 p.
Holmquist, P.J., 1910: Geological Map of the Utö-territory
in the southern coast-regions of Stockholm. Geologiska
Föreningens i Stockholm Förhandlingar 32, pl. 37.
Holmquist, P.J., 1910: Geological Map of the iron-ore-bearing
zone of the island Utö, S.E. from Stockholm, Sweden.
Geologiska Föreningens i Stockholm Förhandlingar 32, pl.
38.
Jonsson, E., 1996: Utös mineralogi. Unpubl. report. Department
of geology and geochemistry, Stockholm university. 7 s.
Koistinen, T., Stephens, M.B., Bogatchev, V., Nordgulen, Ø.,
Wennerström, M. & Korhonen, J., 2001: Geological map
of the Fennoscandian Shield, scale 1:2 000 000. Geological
Surveys of Finland, Norway and Sweden and the NorthWest Department of Natural Resources of Russia.
Lundström, I. & Koyi, H., 2003: Vulkanön som blev ett
fritidsparadis – några glimtar från Utös allra tidigaste
historia. Geologiskt forum 37, 4–13.
Lundström, I., Allen, R.L., Persson, P.-O. & Ripa, M., 1998:
Stratigraphies and depositional ages of Svecofennian,
Palaeoproterozoic metavolcanic rocks in E. Svealand and
Bergslagen, south central Sweden. GFF 120, 315–320.
Smeds, S.A., 1986: de Andrada e Silva, 1763–1838, mineralog
och statsman. Berg-&-dalbladet nr. 4, 3–7.
Talbot, C. J., 2008: Palaeoproterozoic crustal building in NE
Utö, southern Svecofennides, Sweden. GFF 130, 49–70.
J. Mansfeld: The geology of Utö – excursion guide
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